J . Am. Chem. SOC.1982, 104, 5971-5977 since this composition represents the iodinated phase with smallest value of x (ref 2 and see below), and similarly for the TTFI, phase of lowest iodine content. We are also interested in the energetics of the formation of more highly iodinated TTFI, phases, according to the generalized reaction D
+ (~i/2)12
-+
DI,,
('44)
where i > 1. If a cathode is prepared from a mixture of two adjacent line phases DI, and DI,, xk > xj, then the cell with silver anode will give a voltage GEo(kj) corresponding to (xk
- x,)Ag
+ DI,,
-.+ (xk
- x,)AgI
+ DI,
(A5)
E = GEo(kj) and the free energy for the reaction ((xk
- xj)/2)Iz + DIxj
DIx,
('46)
is simply the difference in formation free energies of DI, and DIxk: AG(DI,,) - AG(D1,)
(x, - xk)F(EAgI0 - GE'(kj))
(A7)
Thus, the free energy of formation for any phase of interest can be calculated if the free energies are known for the reaction of
5971
eq A2 and for all transformations between adjacent line phases with lower I content, according to eq A6. The TTFI, phase with x = 2.0-2.3 is not fully treated in the above analysis because it is not a true line phase and because of the observation of two structural modifications within this phase.5 Then, to be rigorous, we should treat each point within the phase range independently. Also, the composition-independent voltage of cells with this as the lower iodine phase means that the last entry in Table I1 should have xj = 2.3. However, the composition range for this quasi-line phase is very narrow, the two reported structures are quite similar, and most important, our experiments indicate that there is a negligible free energy change across the composition range: cells prepared from independent batches of "TTFI," having slightly different stoichiometries showed no discernible differences. Therefore, we treat this phase as a single composition, taking x = 2. This treatment is supported by the binding energy calculations done by Torrance and SilvermanIb for TTFBr,, x = 0.71-0.76. They found less than 0.25% change in the binding energy over the observed phase range. Both TTFBr,, x = 0.71-0.76, and TTFI,, x = 2.0-2.3 (or x = 0.70-0.72), have segregated stacks and about the same ionicity so that, qualitatively at least, we would expect the binding energy to behave similarly in both materials. Registry No. H2(Pc)I, 33606-65-4; TTFIo,,,, 77547-33-2; TTFI,, 64265-34-5; TTFI,, 55492-88-1; (CH3)4NI3, 4337-68-2.
Crystal Structure of Tetrapropylammonium Fluoride Containing Precursor to Fluoride Silicalite Geoffrey D. Price,+' Joseph J. Pluth,+ Joseph V. Smith,*+ J. Michael Bennett,$ and R. Lyle Pattont Contribution from the Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637, and the Union Carbide Corporation, Tarrytown, New York 10591. Received January 8. I982
Abstract: A precursor to fluoride silicalite was crystallized from a hydrothermal system containing silica, tetrapropylammonium ion (TPA'), and fluoride (F).Single-crystal X-ray diffraction data for a twinned crystal (180 X 50 X 50 pm) of the precursor were refined in space group Pnma ( a = 20.044 (2), 6 = 19.918 (4), and c = 13.395 ( 2 ) A). The a axis of one twin component is parallel with the b axis of the other. Combined X-ray intensities (hkl and khl) were separated in a least-squares refinement. The silica framework has the same topology as that of silicalite and ZSM-5 zeolite. A weak negative correlation between the secant of angle (Si-0-Si) and Si-0 is consistent with molecular-orbital models. The TPAF has similar geometry to tetrapropylammonium bromide and lies at the intersection of the 10-ring channels of the framework in a position consistent with a template mechanism of crystallization. The end carbon atoms of propyl groups lie 2.7 and 3.1 8, away from the end carbon atoms of adjacent propyl groups, and there is insufficient room for replacement of propyl by n-butyl. Oblique optical extinction and slight anomalies in high-angle X-ray diffractions indicate monoclinic symmetry, but the sharpness of X-ray powder diffractions limits angular deviations to less than 0.1 from orthogonal geometry. Slight collapse of the framework and positional disorder of the TPAF may be responsible.
Fluoride silicalite is a new microporous silica polymorph,2 similar in some properties to ~ilicalite,~ and the hydrophobic and organophilic nature of these materials may prove commercially important for the removal of organic compounds from wastewater. The framework linkage of silicalite3 is topologically the same as that of synthetic high-silica zeolite ZSM-5,4s5a shape-selective catalyst5-* capable of converting methanol into water and hydrocarbons, useful in internal-combustion engines. The precursor of fluoride silicalite2 crystallizes from a hydrothermal system containing silica, tetrapropylammonium (TPA'), and fluoride (F)ions, and the ideal chemical composition is TPAFSi240,8. Because of the s u g g e ~ t i o n ~ . ~that - " quaternary The University of Chicago. 'Union Carbide. 0002-7863/82/1504-5971$01.25/0
ammonium cations can act as templates around which tetrahedral frameworks assemble during crystallization, the crystal structure (1) Present address: Department of Earth Sciences,Cambridge University, Cambridge, England CB2 3EQ. (2) Flanigen, E. M.; Patton, R. L. US. Patent NM4073865, 1978. (3) Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. L.; Smith, J. V. Nurure (London) 1978, 271, 512-516. (4) Argauer, R. J.; Landolt, G. R. U S . Patent NM3 702 886, 1972. (5) Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. Nurure (London) 1978, 272, 437-438. (6) Kokotailo, G. T.; Meier, W. M. Spec. Publ.-Chem. SOC.1980, 33, 133-139. (7) Meisel, S. L.; McCullough, J. P.; Lechthaler, C. H.; Weisz, P. B. CHEMTECH 1976, 6, 86-89. (8) Naccache, C.; Benn Taarit, Y. In "Proceedings of the Fifth International Conference on Zeolites"; Rees, L. V. C., Ed.; Heyden: London, 1980. 0 1982 American Chemical Society
Price er ai.
5972 J, 4 m . Chem. Soc.. Vol. 104, No. 22, 1982 Y Z
Figure 2. Exploded representation of interpenetrant twinning of a crystal as in Figure la. The central, major twin component is structurally related to the two parallel minor components by a 90” rotation about [OOI]. Directions of the crystallographic axes a, b, and c and the principal axes of the optical indicatrix a.8, and y are shown. The measured V ~ ~ U Cof S the principal refractive indices for Na D light are a 1.491, fl 1.493, and y 1.496. The observed correspondence of the optical /3 and crystallographic D axes is consistent with the report” that the (I axis is the crystallagraphic symmetry axis far a silicalite in space group P2, I 1 and that the crystallographic angle a is not orthagonal.16
Figure 1. Scanning electron micrographs of TPAF-precursor crystals to fluoride silicalite: (a) twinned crystal showing wedge-shaped crystallites on all 1100) faces. scale bar 10 um; (b) rosette of crystals, scale bar 100 wm. Diffraction intensities from the wedge-shaped crystallites were too small to affect seriously the data collection. of the precursor was determined.” Reference will be made to the crystal structure o f silicalite’ obtained by calcination of a precursor synthesized in a non-fluoride system. T h e atomic coordinates thereof are of low accuracy because of (a) the difficulty of collecting and refining X-ray d a t a from the small crystals and (b) the problems caused by interpenetrant twinning and monoclinic, pseudoorthorhombic symmetry.‘) Revised atomic coordinates for this silicalite were presented“ for a refinement based on the pseudo space group Pnma. After completion of the present structure determination of the precursor to fluoride silicalite,12 the detailed crystal structure and structure-related properties of a n as-synthesized ZSM-5 was published by Olson et al.” The ZSM-5 crystal is apparently untwinned, orthorhombic with space group Pnma, cell dimensions a = 20.07 (I). b = 19.92 ( I ) , and c = 13.42 ( I ) A. Its chemical composition is M~II.ISi91.90,P2, where M is a mixture of Na and tetraalkyl cations of unspecified nature whose positions were not located except for two positions labeled 0 x 1 and 0x2. Although the framework topologies of the fluoride silicalite precursor and ZSM-5 a r e the same, there a r e some differences in t h e cell dimensions and atomic coordinates of the frameworks as should h e expected considering the different chemical compositions of the framework and nonframework constituents. Particularly important in the present study of t h e precursor to fluoride silicalite is the location of the T P A F complex within the channels o f the silica framework. Optical and X-ray Study The crystals of the fluoride silicalite precursor used in this study M T U ~ as either interpenetrant twins (Figure la) or polycrystalline m ~ c t t e ~ (Figure Ib). The largcr of the two twin components (Figure 2) has pyramidal boundaries with the two pieces of the second component, and if untwinned, both components would be tablets elongated along e of size (9) Flanigen. E. M. Ad”. Chem. Scr. 1973, No. 121. 119-139. (IO) Aiello. R.;Barrer, R. M.3. Chem. Soe. A 1910, 147W1475. (11) Grase, R. W.;Flanigen, E. M. U.S. Patent NM4061724, 1977. (12) Price, G . D.: Pluth. J. I.: Smith. J. V.; Araki. T.; Bennett, J. M. Norurr (London) 1981, 292.818-819. (13) Bennett, J. M.; Cohen, J. P.; Kirchner, R. M.Am. Crwtallofl. Assa‘. Meel. A b l r . 1979, 6. 62. (14) Smith, J. V. In ‘Fifth International Conference on Zwlitcs-Recent Progress Reports and Discussions”;Ssrsalc, R., Colclla, C., Aicllo, R., Eds.; Heyden: London, 1981; pp 218-225. ( I S ) Olson, D. H.; KokoLlilo, G. T.; Lawton. S. L.; Mcier, W . M.3. Phys. Chem. 1981.85.2238-2243.
-I80 X 50 X 50 wm. Electron microprobe analyses showed only S O 2 and no AI,O, at thc dctection level (
